Aircraft components are subject to high stresses in operation. This can lead to a faulty crack formation, not only in components made of composite materials, such as structural or metallic components, for example landing-gear components, but also in aircraft engine components, in particular. Similar damage patterns are also evident in other gas turbines, such as in stationary gas turbines. The combustion chamber components in gas turbines are highly susceptible to cracking.
Cracks are local material separations within a structure or within a component. Normally, crack initiation is a local event in the microstructure of a surface that is typically caused by lattice defects in the microstructure or by cyclical operational loads. As a rule, cracks propagate orthogonally to the acting normal stress. This propagation is characterized as a normal stress-controlled process.
In the case of combustion chamber components, cracks form due to high thermal and mechanical loads. On the one hand, the prevailing high temperatures cause the cracks to form; on the other hand, the vibrations transmitted to the combustion chamber from the upstream and downstream modules, the high-pressure compressor and the high-pressure turbine promote crack growth and formation. Moreover, short-term thermal material stresses during gas turbine start-up and, as the case may be, during the starting phase of aircraft operation, encourage crack initiation. Solid particles, such as sand and dust, which are drawn into the gas turbine, are likewise highly conducive to crack initiation in the combustion chamber components. Moreover, during the gas turbine's operating phase, sustained thermal stresses induce a change in the geometric shape of the combustion chamber components.
The main problem associated with maintaining and servicing aircraft and/or gas turbine components, particularly with maintaining and servicing the combustion chamber, is detecting the cracks that form and the geometric variations that arise during operation, and repairing the components through appropriate measures. This is often difficult to accomplish due to the unique characteristics of the particular cracks or damage.
Known methods for detecting cracks include a number of non-destructive testing procedures. Current methods include the dye penetration method, ultrasonic testing, eddy-current testing, X-ray testing and magnetic powder testing, for example.
In the majority of components, in particular in aircraft engine components, such as combustion chamber components, for example, the dye penetration method is used to test for cracks.
The dye penetration method usually encompasses at least the following five steps:
1. precleaning and drying the test specimen to remove accumulated dirt from between the crack flanks;
2. applying what is generally referred to as the penetrant (penetrating oil having fluorescent pigments) that penetrates into the cracks;
3. intermediate drying (removing the excess penetrant) and drying;
4. applying what is generally referred to as the developer (chalk-based powder) to make the cracks visible;
5. manual analysis of the indications under ultraviolet light by trained personnel. The indications (for example, cracks) are marked on the test specimen.
Following the dye penetration test, the test specimen undergoes a visual control. A manual inspection is performed to check whether the indications are actually cracks and whether the marked crack indications are within the permissible tolerance. The tolerances for the component in question are specified in the technical maintenance and service documentation.
Following the visual control, geometric measurements of the test specimen are likewise taken manually. The dimensions and measuring positions to be checked are defined in the technical service documentation. Trained specialists measure the test specimen using measuring implements, such as a special measuring tape, for example, and the exact geometric data are subsequently documented.
The dye penetration method actually used for the components is a manual method that is quite time-consuming. Due to its many process steps, the dye penetration test substantially influences the process and processing time needed for component maintenance. Moreover, test reproducibility is not or at least not fully given as the indications are analyzed manually. The test quality is influenced by the human factor. Moreover, dye penetration is a chemical and energy-intensive test procedure. Therefore, it has an environmental impact.